It is well-known in the art that zinc oxide (ZnO) blocks ultraviolet (UV) radiation at wavelengths from 290 nm up to about 375 nm. In addition, zinc oxide has long been utilized for its antimicrobial and other properties. Despite these beneficial properties, use of zinc oxide has been limited primarily due to an undesirable whitening effect on the substrate to which a zinc oxide-containing product was applied. To the extent that zinc oxide was incorporated into dispersions for cosmetic and sunscreen formulations and products, formulators minimized ZnO levels and/or users applied the product sparingly or at levels lower than indicated to reduce or minimize whitening. In so doing, however, the photoprotective efficacy of the product was lessened. Similarly, such whitening was and is undesirable in photoprotective transparent coatings and transparent plastic films.
Whitening on a substrate (e.g., skin) after application of a photoprotective product containing dispersed ZnO powder is attributable to scattering of light from the particles in the backward direction (i.e., away from the substrate and toward the viewer). In contrast, light scattered in the forward direction (through the substrate) contributes to the transmittance of light. This is known in the art as “diffuse” transmittance. Total transmittance of incident light through a ZnO-containing photoprotective product is thus comprised of light that is diffusely transmitted as well as light that is transmitted without scattering, known in the art as “specular” transmittance.
The main factors that affect the scattering of light from particles and hence whitening include the particle size and the refractive index of the particles relative to the media in which the particles are dispersed. In general, decreasing the size of the particles or the relative refractive index of the particles causes a decrease in scattering and whiteness of the product.
Prior art approaches to the problem of surface whitening caused by ZnO-containing photoprotective products have concentrated largely on reducing the average size of the zinc oxide particles in the product to below at least 0.2 micrometers. This particle size reduction decreases the scattering of light from the particle surfaces which increases transparency and reduces whiteness. For example, U.S. Pat. No. 5,587,148 teaches a substantially visibly transparent topical sunblock formulation comprising a dispersion of micronized particles of zinc oxide having an average particle diameter of less than about 0.2 micrometers. U.S. Pat. No. 5,032,390 teaches sunblock compositions comprising from 1% to 25% by weight of particulate zinc oxide having an average particle size of from 0.07 microns to 0.3 microns. The disclosed compositions are further taught to include from 1% to 25% of particulate titanium dioxide having an average particle size of from 0.03 microns to 0.07 microns.
Reduction of ZnO particle size to nanoscale (e.g., particularly less than about 0.1 microns) is not, however, without consequences. Nanosize ZnO particles have been associated with a high level of photocatalytic activity associated with the formation of free radicals and, resulting in degradation of polymeric ingredients typically contained in cosmetics, plastics, and paints. Moreover, in photoprotective personal care products, high photocatalytic activity can produce free radicals which have been reported to cause deleterious health effects.
The greater available surface area of nano-sized particles may increase the amount of flocculation and, in turn, agglomeration. Photoprotective products containing nanosized particles therefore may be unstable and, in the case of emulsions, phase separation. This instability can lead to higher scattering of light and increased whiteness than would otherwise be expected based on particle size alone, as well as a reduced level of photoprotection.
Recently, concerns have been raised regarding potential negative health consequences of transdermal penetration of nano size inorganic particles and systemic absorption of organic sunscreen filters and their breakdown products. Irrespective of whether or the extent to which these concerns are substantiated, there has been and remains an as yet unmet need for topical photoprotective compositions that minimize or, preferably, do not contain organic sunscreen filters and/or nano-sized physical sunscreen blocking agents. There remains a need for zinc oxide powders, that are of a sufficiently large size to not raise concerns about product safety or stability, that when dispersed in a transparent matrix provide substantial visible transparency combined with minimal or no whitening.
Alternative to reducing particle size, the intensity of light scattered at particle interfaces can be decreased by reducing the difference in the refractive index across the interface. For example, nanoporous films and coatings are known to exhibit improved transparency and reduced reflection associated with the reduction in the relative refractive index associated with the film structure. For example, Hiller et al (Nature Materials, 2002, 1, 59-63) describe nanoporous polymer films with increased light transmission and reduced reflection. U.S. Pat. No. 7,075,229 teaches a light-emitting device incorporating a transparent nanoporous alumina film. US Patent Publication Application No. 2006/0188432 teaches a method of producing porous titanium oxide powder with improved transparency.
There has been and there remains a need for transparency to be achieved using a dispersion of zinc oxide powders that are not predominately comprised of nano-sized particles. This need is met by embodiments of the present invention.